This invention relates to improved processes for preparing titanium dioxide pigments by multistage continuous wet treatment characterized by short reaction times at low temperatures. Finished pigments prepared by the present processes are useful in various applications (i.e. paint, plastic coatings, etc.) and possess improved properties such as gloss and/or durability.
Titanium dioxide (TiO2) pigments are useful in a variety of applications such as paints, plastics and coatings. Generally pigments useful in these applications contain base TiO2 particles (i.e., anatase or rutile) having layers or coatings of hydrous oxides of alumina, silica, zirconia, phosphates and the like.
Processes used to coat TiO2 are well known in the pigmentary arts. These coating processes typically involve wet treating TiO2 particles by precipitating certain hydrous oxides to form the desired coating.
Wet treatment of TiO2 is performed using either batch processes or continuous multi-stage processes. Some prior art references describe advantages to continuous processes including reduced treatment times for hydrous oxide coating, and high product throughput without the need for substantial capital investment.
It is well known in the art that pH, temperature and wet treatment times performed on TiO2 must be carefully controlled to produce fully coated pigment particles having desired physical properties (i.e., gloss and/or durability). Of the hydrous oxides, combinations of silica and alumina have proven to possess excellent properties in coatings applications. For example, pigments having dense silica coatings are well known to influence the optical properties and improve the durability of plastic, paint, or coating that it is used.
Continuous multistage processes to coat TiO2 particles with hydrous silica and alumina are described in the prior art. Typically, these processes involve preparing an aqueous slurry of TiO2 particles at temperatures of at least 80xc2x0 C. and subsequently adding soluble silicates and alumina to the slurry.
The pH of the slurry is adjusted to at least about 9.6 to initiate the deposition of the hydrous silica layer. The prior art describes that in order to produce pigments with desirable silica coating, excessively long treatment times (i.e., over one hour or longer) and specific pH ranges are needed. For example, adequate dense silica coating is obtainable when slurry pH is lowered to about 9 through a series of steps designed to control the curing of the silica.
The prior art discloses that long treatment times and slurry pH are important to alumina deposition. For example, after curing the silica over one hour, alumina deposition is initiated by lowering the slurry pH to between about 5.5 and 7.5 by the addition of an alumina compound such as sodium aluminate. The reaction is allowed to proceed until such time as the desired amount of alumina has been deposited. Subsequently, the slurry is neutralized and washed. In a separate finishing step, the resulting pigment filter cake is finished by known means.
It has been discovered that in order to prepare TiO2 pigments for paints, plastic and coatings having acceptable durability and/or gloss, that excessively long treatment times and higher treatment temperatures, typically greater than about 80xc2x0 C. are unnecessary. These temperatures consume excessive energy, and are deleterious to the process. Thus, the present invention represents advancements in the art of continuous processes producing TiO2 pigments having hydrous oxide coatings.
The present invention provides a continuous process for preparing titanium dioxide pigment comprising: a) preparing an aqueous slurry of titanium dioxide base particles at a temperature below about 75xc2x0 C. and at a pH of from about 3.0 to about 9.0; b) adding a silica compound while maintaining the pH of the slurry in the range of from about 2.4 to about 10.5 and the temperature of the slurry below about 75xc2x0 C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75xc2x0 C. so as to form the titanium dioxide pigment.
In one embodiment, the present invention provides a continuous process for preparing a titanium dioxide pigment comprising: a) preparing an aqueous slurry of titanium dioxide base particles at a temperature below about 75xc2x0 C. and at a pH from about 3.0 to about 8.0; b) adding a silica compound to coat the base titanium dioxide particles by maintaining the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 10.0, in the second stage the pH is maintained at about 9.4, and in the third stage the pH is maintained at about 4.9, while maintaining the temperature of the slurry below about 75xc2x0 C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75xc2x0 C. so as to form the titanium dioxide pigment.
In another embodiment, the present invention provides a continuous process for preparing a titanium dioxide pigment comprising: a) preparing an aqueous slurry of titanium dioxide base particles at a temperature below about 75xc2x0 C. and at a pH from about 3.0 to about 8.0; b) adding a silica compound to coat the base titanium dioxide particles by adjusting the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 9.6, in the second stage the pH is maintained at about 8.9, and in the third stage the pH is maintained at about 2.7, while maintaining the temperature of the slurry below about 75xc2x0 C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75xc2x0 C. so as to form the titanium dioxide pigment.
In another embodiment, the present invention provides a continuous process for preparing a titanium dioxide pigment comprising: a) preparing an aqueous slurry of titanium dioxide base particles at a temperature below about 75xc2x0 C. and at a pH from about 3.0 to about 8.0; b) adding a silica compound to coat the base titanium dioxide particles by adjusting the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 9.5, in the second stage the pH is maintained at about 9.2, and in the third stage the pH is maintained at about 6.1, while maintaining the temperature of the slurry below about 75xc2x0 C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75xc2x0 C. so as to form the titanium dioxide pigment.
In yet another embodiment, the present invention provides a continuous process for preparing a titanium dioxide pigment comprising: a) preparing an aqueous slurry of unmilled titanium dioxide base particles at a temperature below about 75xc2x0 C. and at a pH from about 2.0 to about 9.0; b) adding a silica compound to coat the base titanium dioxide particles by maintaining the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 9.4, in the second stage the pH is maintained at about 9.1, and in the third stage the pH is maintained at about 5.0, while maintaining the temperature of the slurry below about 75xc2x0 C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75xc2x0 C. so as to form the titanium dioxide pigment.
In one exemplary embodiment, the present invention provides a continuous process for preparing a titanium dioxide pigment comprising: a) preparing an aqueous slurry of wet milled titanium dioxide base particles at a temperature below about 75xc2x0 C. and at a pH from about 2.0 to about 9.0; b) adding a silica compound to coat the base titanium dioxide particles by maintaining the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 9.1, in the second stage the pH is maintained at about 8.6, and in the third stage the pH is maintained at about 5.4, while maintaining the temperature of the slurry below about 75xc2x0 C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.4 to about 7.5 and the temperature of the slurry below about 75xc2x0 C. so as to form the titanium dioxide pigment.
In another exemplary embodiment, the present invention provides a continuous process for preparing titanium dioxide pigment comprising: a) preparing an aqueous slurry of titanium dioxide base particles at a temperature below about 75xc2x0 C. and at a pH from about 3.0 to about 8.0; b) adding a silica compound to coat the base titanium dioxide particles by maintaining the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 10.0, in the second stage the pH is maintained at about 9.4, and in the third stage the pH is maintained at about 4.9, while maintaining the temperature of the slurry below about 75xc2x0 C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75xc2x0 C. so as to form the titanium dioxide pigment.
The pigments of the present invention can be prepared in a minimal amount of time, utilizing less overall heat energy than other continuous processes. The pigments of the present invention will have excellent gloss potential and compatibility with paints, plastics or coatings.
For a better understanding of the present invention together with other and further advantages and embodiments, reference is made to the following description taken in conjunction with the examples, the scope of which is set forth in the appended claims.
The invention will now be described in connection with preferred embodiments. These embodiments are presented to aid understanding the present invention and are not intended to, and should not be construed to, limit the invention in any way. All alternatives, modifications and equivalents that may become obvious to those of ordinary skill on reading the disclosure are included within the spirit and scope of the present invention.
This disclosure is not a primer on TiO2 pigment production, basic concepts known to those skilled in the field of TiO2 production have not been set forth in detail. Concepts such as choosing appropriate additives and reactors for the oxidation reaction producing titania pigment are readily determinable by those skilled in the industry and are generally described in the prior art. Attention is therefore directed to the appropriate texts and references known to those skilled in the art in regard to these matters.
With a view to the present inventive process, TiO2 pigment bases may be utilized. Such bases may include those commercially manufactured by either the xe2x80x9cchloridexe2x80x9d or xe2x80x9csulfatexe2x80x9d process. The TiO2 base particles useful in the present invention should preferably possess a substantially rutile crystalline structure. Such a base made by, for example, the chloride process will have a particle size range of from about 0.1 to about 0.5 microns. Typically, bases made via the chloride process contain a small amount of aluminum oxide which is added during the chloride process as, for example, aluminum chloride. The aluminum compound may be present in the TiO2 pigment base in an amount of from about 0.1 to about 1.5 percent based on the titanium compound as is typically known in the art.
The present invention includes TiO2 base obtained from the sulfate process, where titanium ore is dissolved in sulfuric acid to prepare an aqueous titanyl sulfate. This aqueous titanyl sulfate is hydrolyzed to titanium hydroxide and subsequently calcined to produce the base TiO2. Both of these processes are well known in the art.
The base TiO2 pigments useful in the processes of this invention can be wet milled, dry milled or unmilled and optionally hydroclassified prior to treatment, providing a pigment substrate of substantially uniform particle size.
As used herein unmilled base TiO2 is an art recognized term and includes TiO2 pigment that is not wet milled. Unmilled base TiO2 pigment includes crude TiO2 discharge product from the reactor in the chloride process or calciner in the sulfate process, where it was produced, which has not been subjected to any prior intervening processing step that substantially grinds, crushes, or mills the discharge product. Dry milled base TiO2 is an art recognized term and includes dry milled or rolled TiO2 discharge product from suitable dry milling or rolling processes, such as for example, Raymond milling, Sahout Conreur or Fitzpatrick rolling, and the like. Wet milling is an art recognized term and includes milling wet base TiO2 discharge product. Some wet milling processes include sand milling, zircon bead milling, and the like.
The process of the present invention includes formation of an aqueous slurry of base titanium oxide pigment. Slurries of titanium dioxide pigment can be made by methods known in the art. Typically, the base pigment concentration will range from about 100 to about 500 grams per liter of slurry. This slurry is heated to a temperature of between about 60 to 75xc2x0 C. by any means known in the art. Preferably, the slurry is heated to a temperature of between about 65 to below about 75xc2x0 C. via steam injection, being fed in a continuous manner into the process reactor system.
The slurry pH can be adjusted by methods known in the art. For example, adjustment of pH is accomplished by adding a suitable acid or suitable base. Suitable bases include water-soluble alkaline bases such as ammonia, sodium hydroxide or other suitable alkaline compounds. Suitable acids include water soluble acids such as hydrochloric acid, sulfuric acid, nitric acid, and the like. The pH of the initial slurry (prior to addition of the silica compound) can be adjusted to between about 2 and about 11. Preferred pH of the initial slurry is between about 2 and about 9, most preferably from about 3 to about 8.
Following the formation of the initial slurry, a silica compound is added while the slurry is maintained at a temperature of below about 75xc2x0 C. For purposes of the present invention, any water-soluble silica compound capable of providing silica for deposition upon the titanium dioxide pigment under the operating conditions of the process can be employed. Silica compounds suitable for use in the present invention include, but are not limited to, water-soluble alkali metal silicates. Preferred water-soluble alkali metal silicates include sodium silicate, potassium silicate, and the like. Most preferably, the silica compound is aqueous sodium silicate that provides SiO2.
The silica compound is typically added directly into the reaction vessel in which the wet treatment takes place, or it can be added inline to the process. Preferably, the silica is added in an amount of from about 1 to about 10 weight percent, more preferably from about 1.5 to about 5.5 percent and most preferably from about 2.5 to about 3.5 percent, based on the weight of the TiO2 pigment.
After the silica addition, the slurry is cured at pH ranges from typically about 10.5 to about 2.4. Preferably, the slurry is cured in at least three stages, for example, the pH is lowered in the first stage from about 10.2 to about 10.0, in the second stage, the pH is lowered to about 9.4, then in the third stage to about 4.9. Another preferred pH range includes lowering the pH in the first stage from about 10 to about 9.6, in the second stage, the pH is lowered to about 8.9, then in the third stage the pH is lowered to about 2.7. Another preferred pH range includes lowering the pH in the first stage from about 9.7 to about 9.5, in the second stage, the pH is lowered to about 9.2, then in the third stage the pH is lowered to about 6.1. Another preferred pH range includes lowering the pH in the first stage from about 9.6 to about 9.4, in the second stage, the pH is lowered to about 9.1, then in the third stage the pH is lowered to about 5.0. Another preferred pH range includes lowering the pH in the first stage from about 9.5 to about 9.1, in the second stage, the pH is lowered to about 8.6, then in the third stage the pH is lowered to about 5.4. The present invention contemplates similar pH profiles as well.
The residence time at each stage may also vary. Preferably, the residence time is kept to a minimum period of preferably about 15 minutes. For example, a sufficient amount of silica compound is added over 10 minutes, then the slurry is held for an additional period (preferably 5 minutes) prior to lowering the pH with each addition of acid. Alternatively, the silica compound is added in less than a minute, then the slurry is held for about 15 minutes prior to lowering the pH with additions of acid.
In one contemplated preferred embodiment, the present invention provides a continuous process where the slurry pH is between about 9.6 and about 9.8 in a first silica precipitation vessel. The slurry is allowed to cure where the residence time is preferably between about 10 and about 20 minutes prior to transferring the slurry to a second silica precipitation tank. In the second tank, the pH is lowered and maintained at between about 9.2 and 9.5. The residence time in the second tank is between about 10 and about 20 minutes prior to transferring the slurry to a third silica precipitation tank. In the third silica precipitation tank, additional acid is added to maintain the pH at a range of from about 2.4 to about 6.6.
Following the silica addition, an alumina compound is added while the slurry is maintained at a temperature of below about 75xc2x0 C. Alumina compounds include hydrous alumina compounds such as for example, water soluble alkali metal aluminates. Some water soluble alkali metal aluminates, include but are not limited to sodium aluminate or potassium aluminate. Some other alumina compounds include aluminum sulfate, aluminum chloride, and the like. Most preferably, the water soluble alumina compound is sodium aluminate that provides Al2O3.
Preferably, the alumina compound is added to the slurry to provide Al2O3 equivalent to between about 1 and about 3% alumina by weight based on the weight of the titanium dioxide base pigment. Most preferably, the Al2O3 is equivalent to between about 1.6 to about 1.9% alumina based on the weight of the titanium dioxide base pigment.
Typically, the alumina compound is added to the slurry over between about 10 to about 20 minutes with simultaneous additions of either acid or base to maintain the pH below 7. Alternatively, the alumina compound is added in less than a minute, with simultaneous additions of either acid or base to maintain the pH below 7. Preferably, the pH is maintained at between about 5.5 and about 6.5 and the slurry is held for about 15 minutes prior to being discharged to a filter feed tank or directly to filters for washing.
The resultant silica and alumina coated titanium dioxide pigment is washed substantially free of soluble salts adhering to the pigment, dried and then subjected to final comminution using fluid energy milling techniques known in the art. Preferably, the washed and dried pigment is micronized in a steam micronizer at intensities known by those skilled in the art to produce the desired particle size distribution.
Optionally, an organic compound, polyol-like such as, for example, trimethylolpropane (TMP), trimethylolethane (TME), pentaerythritol, Tamol 1254, Tamol 963 and hydrophobic organics like organic phosphoric acids and silanes, can be added to the pigment during air or steam micronization. In the most preferred embodiment, TMP can be added in an amount of from about 0.2% to 0.8% based on the weight of the titanium dioxide pigment.
Titanium dioxide pigments of the present invention may be produced by a continuous process. Typically, continuous processes involve continuous feed pipelines with cascading treatment tanks with separate addition points for silica and alumina compounds, pH adjusters and other additives. Continuous processes involve residence times of less than about 120 minutes, preferably from about 5 minutes to about 60 minutes, and more preferably from about 10 minutes to about 45 minutes for silica and alumina deposition on the titanium dioxide pigment.
The coated pigments produced by the methods of the present invention will have good, dispersibility, gloss and/or durability. Dispersibility is determined by methods known in the art. For example, the coated titanium dioxide pigments of the present invention can be mixed in a plastic or paint and the distribution of the pigment particles measured. Uniform distribution of the pigment throughout the paint or plastic indicates good dispersibility, while agglomerate formation would indicate poor dispersibility of the pigment. Some methods of determining dispersibility known in the art include tinting strength, Hegman gauge, and the like.
Gloss is determined by methods known in the art. Preferably, the gloss is determined by incorporating the pigment into paint and measuring the gloss using a gloss meter. Durability of the pigments of the present invention can be determined by methods known in the art. Some methods of measuring durability include measuring photocatalytic activity of the titanium dioxide pigment, acid solubility of the titanium dioxide pigment, natural exposure and weatherometer testing.
Having now generally described the invention, the same may be more readily understood through the following reference to the examples, which are provided by way of illustration and are not intended to limit the present invention unless specified.